Transmission system



Feb. 1 1927. 1,615,967

E. vRooM TRANSMISS I ON SYSTEM Filed Nov. 19, 1924 Patented Feb. 1, 1927.

UNITED STATES 1,615,967 PATENT oFFlcE..

EDWARD VROOM, OIF OSSINING, NEW YORK, ASSIGNOR TO WESTERN ELECTRIC COllll'.` PANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION 0F NEW YORK.

TRANSMISSION SYSTEM.

l Application led November 19, 1924. Serial No. 750,739.

This invention relates to transmission systems, and is particularly adapted tosystems employing repeaters.

An object of the invention is to improve the transmission characteristics of a twowire transmission system.

Another object of theinvention is to reduce the cost and facilitate the maintenance of such a system.

A related object of the invention is to eliminate balancing equipment at repeater points in a twowire signaling system without impairing the efficiency of the system for two-way communication.

The invention will be described as applied to a two-wire telephone system, but it will be understood that it may also be applied to telegraph and other signaling systems.

It is customary in voice frequency telcphone systems employing one or more rcpeaters to'couplc the repeaters to the transmission line by means of hybrid coils. and associated line balancing networks which render the two-way repeating channels conjiwate.

uch balancing equipment has the disadvantage of being very expensive to manufac-v ture and maintain, and in practice it is difiicult, if not impossible, to maintain a line balance under changing conditions of weather, temperature, etc.

This is particularly true in the case of lower grade circuits where, due to the varied make up of the transmission lines, it is impossible to maintain efficient transmission.

Furthermore, the presence of each hybrid coil and associa-ted balancing network introduces an over-all transmisslon loss of several standard cable miles. i l

This invention does away with the expensive hybrid coils heretofore employed at repeater points in two-wire signaling systems, as well as the transmission problems encountered in their use.

Briefly, the invention comprises a twowire transmission system employing a voice frequency channel for transmissionvin one direction and a carrier frequency channel for transmission 'in the opposite direction. Means are provided at the terminals for modulating and demodulating the carrier wave, and directional filters are employed at each repeater to direct the'signals transmitted in both directions through a single amplifier, thereby segregating the frequencies without the use of balancing networks. The various features andA advantages of s the invention will appear from the following description and the accompanying drawing which is a diagrammatic illustration of a two-wire telephone system embodying the invention.

The drawing represents a two-wire telephone system including west and east terminal stations and a plurality of repeaters R, and R2 interconnected by the transmission line ML.

The terminal apparatus at the west station includes a transmitting channel TC1 and a receiving channel RC1, and the terminal apparatus at the east station includes a transmitting channel TG2 and a receiving channel R 2.

Voice frequencies are utilized for transmission over the line ML from the west station to the east station, while voice modulated carrier frequencies are utilized for transmission in the opposite directions from the east station to the west station.

A low pass grouping filter LP is paired with ahigh pass grouping filter HP at each of the terminal stations. These filters serve to separate the directional bands of fre` quencies to the respective terminal transmitting and receiving channels. These filters and each of the other filters shown throughout the system may be designed in accordance with the principles set forth in the United States Patent to Campbell No. 1,227,- 113, issued May 22, 1917.

A low frequency line L1, which may be an ordinary subscribers telephone line, is associated with channel TG1- RC1 at the west station for communication over the line ML with a similar low fre uency line L2 associated with channel T -RC2 at the east station.

The low frequency lines L1 and L2 are provided with balancing artificial lines or networks N1 and N2, respectively, and with differential repeating coils H1 and H2, commonly knownA as hybrid coils, for enabling independent transmission in the two directions between the lines and the terminal circuits. v

Voice frequency currents originating in the low frequency line L1 at the west station pass through the associated hybrid coil ffl H1 into the transmitting channel TC1 and thence through the low pass grouping filter LP to the line ML. These currents are amplified at repeaters R1 and R2 in a manner hereinafter to be described, and are picked up by the low ass grouping filter LP at the east station and transmitted through channel RC2 and 'hybrid coil H2 to the low frequency lille L2.

For transmission in the opposite direction, however, 'means are provided at the east station for transmitting the lou7 frequency currents originating in low frequency line L2 as modulations of a current of carrier frequency, and for converting the carrier waves received at the West station into currents of audible frequency for transmission to the line L1.

For this purpose transmitting channel TC2 at the east station includes a modulator M and a source of carrier current C, and the channel RC1 at the west station includes a demodulator D.

The modulator M, oscillator C and demodulator D may be of any suitable type such as thel thermionic vacuum tube type of apparatus shown in Fig. 9 of an article entitled Carrier current telephony and telegraphy, by Messrs. Colpitts and Black- Well, published iii the Proceedings of the American Institute of Electrical Engineers, vol. 40, 1921.

Voice frequency currents originating in the low frequency line L2 at the east Sta tion pass through the associated hybrid coil v H2 into the modulator M in the transmitting channel TG2. Carrier current from the oscillator O is likewise fed into the modulator M wherein it is modulated by the voice currents from the' line L1. Of the components' of modulation appearing in the output circuit of the modulator M, the high pass filter HP suppresses all but the carrier and one side band, for example the upper side band, which it transmits or passes to the transmission line ML. These currents are transmitted through repeaters R2 and R1 in a manner hereinafter to be described to the West station.

Currents incoming at the West station pass through the high pass grouping filter HP to the receiving channel RC1', and are demodulated in the demodulator D. Voice frequency components of demodulation appearing in the output of demodulator D are transmitted through hybrid coil H1 to the low frequency line L1.

The repeaters R1 and R2 are identical, and it is to be understood that any number of repeaters may be employed as required.

As described above, the currents transmitted from the West station are of voice frequency, While the currents transmitted from the east station are of carrier frequency. Each repeater consists. of one-way LPIF designed to pass voice frequency curi,

rents, and a high pass input filter HPIF designed to pass carrier frequency currents. A comm-on amplifier A has its input circuit connected to the upper branch of the repeat-er at a point intermediate the 10W pass input filter LPIF and the high pass input filter HPIF. The output circuit of this amplifier is connected to the lower branch of the repeater which includes a high pass output lter HPCF and a low pass output filter LPOF.

Currents of voice frequency incoming .at a repeater from the West station pass through the low pass input filter LPIF, are amplified in the common amplifier A and transmitted through low pass output filter LPOF to the line ML. The carrier frequency currents transmitted from the cast station pass through the high pass input filter HPIF, are amplified in the common amplifier A and are transmitted through high pass output filter HPOF to the Out-v going line.

lt will be seen that both voice and carrier currents transmitted from opposite terminals are directed through the common repeater amplifier b means of the respective repeater directional filters, and after being amplified are segregated and passed on to the respective outgoing line sections through the respective output filters. With properly designed filters none of the output can enter the input of the repeater and cause it to sing.

The carrier frequency employed for transmission from east to' West may be Well under 10,000 cycles, thus avoiding the necessity for special line transpositions which are frequently required in systems employing the higher frequency carrier currents.

The invention set forth herein, of course, is susceptible of various other modification and adaptations Within the scope of the appended claims.

W'hat is claimed is:

l. A two-Way communicating system comprising a transmission line, means for transmitting signaling currents Within the voice range over said line in one direction, means for transmitting signaling currents above the essential voice frequencies over said line in the opposite direction, and a repeater associated with said line and including a common' amplifier for the currents transmitted in both directions said two means for transmitting said currents of different frequencies forming the two sides of a two-way communication channel.

2. A two-way communication system comprising two terminals connected by a transmission line, means at one terminal for impressing waves of speech frequency representing one side of a conversation upon said line`means at said terminal for receiving and detecting carrier waves modulated in accordance with waves of speech frequency and representing the other side of said conversation, means at the other terminal for iinpressing upon said line carrier waves modulated in accordance with waves of speech frequency for transmission to said iii-st station, means at said second mentioned terminal for receiving waves of speech frequency from said first station, and a repeatf er associated with said line and including a common amplifier for the waves transmitted from each terminal.

A two-Way communicating system comprising two terminals connected by a transmission line, means at one terminal for impressing waves of speech frequency upon said line, means at said terminal for receiving and detecting carrier waves modulated in accordance with waves of speech frequency, means at the other terminal for impressing upon said liiie carrier waves modulated in accordance with waves of speech frequency, means at said other terminal for receiving waves of speech frequency, a repeater including an amplifying element associated with said line, a selective path through said amplifying element for the waves of speech frequency, and a selective path through said amplifying element for the waves modulated in accordance with the waves of speech frequency, said means for transmitting .and receiving said waves of respectively ditferentfrequency range forming the two sides of a communication channel.

4. The method of transmission through a system having repeaters, comprising transmitting signaling currents within the voice range representing one side of a conversation in `one direction through the system, transmitting signaling currents above the vessential voice frequencies and representing the other side of said conversation in the opposite direction through the system,

and selectively transmitting said directional frequencies through the same repeater amplifying elements.

5. The method of transmission through a system having repeaters, comprising transmitting speech waves representing one side of a conversation in one direction through the system at the speech-frequency level, transmitting speech representing the other side of said conversation in the opposite direction through the system in the form of modulations` of a carrier wave whereby the modulated waves are at a higher frequency level than the speech frequency level, separating the oppositely directed transmissions at a repeater point by frequency selection, amplifying the transmissions and separately transmitting the amplified transmissions through the system each in its respective direction.

6. A transmission system comprising a main transmission line and terminal circuits, said terminal circuits being traversed by two-way conversation currents, means for impressing voice frequency currents from one such terminal circuiton said main line for transmission thereover and means at the other terminal for impressing voice frcquency currents received over said main line upon the terminal circuit thereat, means at the last mentioned terminal for converting` voice waves from raid terminal circuit into modulated side band components having frcquencies above the frequency range of tlic speech currents transmitted over the main line, .and for impressin' said side band on said main line, means at the opposite terminal. for demodulating the side band waves transmitted over the main line to produce speech frequency currents and for impresing the. demodulated speech frequency waves on theterininal circuit thereat, a repeat-l ing station on said main line containing an amplifying repeater, and band Vfilters at said terminal stations and at said repeating station for separating the speech frequency waves traversing the main line from the side band frequencies to enable non-interfering transmission and reception and ampliication of the oppositely directed transmissions on said main line.

In witness whereof, I hereunto subscribe my name this 14th day of November A. D., 1924.

- EDWARD VROOM. 

